She has been cheating?

Few days ago me & my partner were on her facebook she went into her inbox & I saw some messages off men that looked a bit dodgy so I asked to read them which she accepted and I was shocked at what I read in these conversations she was basically cybering with this man also arranging meet ups with them for sex which she denied going to she said she felt a bit naughty & that their just mates..so that's why she did it I'm at a point where I don't know wheher to cut and run or stay?